With the recent progress in machine technology, there are growing desires for size reduction, weight reduction, precision increase, life prolongation, etc. in machines. Since the joints, bearings, gears, and other components of rotating parts also are small-sized and operated under high-speed and high load conditions, the atmospheres in which the lubricating greases applied to such parts are used have been becoming very severe.
CVJs (CV joints) and steel-rolling machines are taken as examples to explain the above in more detail.
In the automobile industry, the number of vehicles employing a CVJ has increased with the increase in the number of FF (front engine front drive) automobiles. Not only FF vehicles but also four wheel drive (4 WD) vehicles are increasing in number recently, with which the amount of CVJs for automotive use increased rapidly. In particular, because of the trends toward power and performance increase in FF vehicles and toward size reduction and weight reduction in CVJs and because operating condition of CVJs are becoming more severe, the durability requirement for CVJs is becoming more and more severe. For example, CVJs have come to be disposed at increased angles and be operated at higher speeds under higher loads due to the employment of turbo-equipped or larger-sized engines and, hence, there are cases where the temperature of CVJs rises rapidly during driving because of, e.g., increased internal heat generation. Various kinds of CVJs exist, which are properly used according to applications. Since the lubricants to be applied to CVJs are also required to cope with torque and speed increase, there is a desire for a grease which not only has excellent resistance to higher temperatures but also is excellent in so-called heating-inhibitory effect, i.e., the effect of diminishing the friction of sliding parts to minimize temperature increase.
The inhibition of temperature increase by the diminution of friction is desired also from the standpoints of improving the durability of joints and sealing boot materials and retarding the deterioration of the lubricant itself. An excessive temperature increase accelerates the aging of the sealing boot material and the deterioration of the lubricant, resulting in a significantly shortened CVJ life.
In the steel industry, on the other hand, there has been a strong desire for greases with higher qualities such as longer life and higher heat resistance because of the necessity for energy saving, labor saving, resource saving, and prevention of environmental pollution. A steel factory contains various kinds of machinery, and greases to be used therein slightly differ in required performance depending on the atmospheric conditions. In the steel rolling step, in which most of the greases are consumed, the bearings, sliding surfaces, screws, and other parts of the rolling machine are greased by means of central lubrication, and the greases for this use mostly contain an extreme pressure additive. Since such mechanical parts in the steelmaking equipment are considerably affected by load and heat and are operated in an environment containing water and scales, a grease excellent especially in wear resistance, frictional property, and sealing property is desired for the elongation of the lives of these mechanical parts.
In order to cope with the above-described desires, extreme pressure lithium greases are mainly used in the market. These greases contain a sulfur-phosphorus extreme pressure additive comprising a combination of a sulfurized oil, fat, or olefin and zinc dithiophosphate, a lead compound additive, and molybdenum disulfide. Further, urea greases having better heat resistance than the lithium greases are recently being used increasingly.
Under these circumstances, representative prior art techniques include U.S. Pat. Nos. 4,840,740 and 4,514,312 and JP-B-4-34590. (The term "JP-B" as used herein means an "examined Japanese patent publication.") U.S. Pat. No. 4,840,740 discloses a urea grease containing as an additive a combination of an organomolybdenum compound and zinc dithiophosphateo. U.S. Pat. No. 4,514,312 discloses a urea grease containing an aromatic amine phosphate. Further, JP-B-4-34590 discloses a urea grease containing as an essential ingredient a sulfur-phosphorus extreme pressure additive comprising a combination of (A) a sulfurized molybdenum dialkyldithiocarbamate and (B) at least one selected from the group consisting of sulfurized oils or fats, sulfurized olefins, tricresyl phosphate, trialkyl thiophosphates, and zinc dialkyldithiophosphates.
However, the greases according to these prior art techniques have a problem that they deteriorate sealing materials. That is, the sealing boot materials, which mostly are chloroprene rubbers, silicone rubbers, and polyester resins, are deteriorated by the conventional greases at high temperatures. For example, greases containing such additives as a sulfurized oil or fat and a sulfurized olefin deteriorate chloroprene rubber to cause considerable changes in tensile strength and elongation. Greases containing a zinc dialkyldithiophosphate deteriorate silicone rubbers, while greases containing lead naphthenate accelerate the deterioration of silicone rubbers and polyester resins to greatly affect the properties thereof.